This application generally relates to hard disc drives and more particularly to an apparatus and method for optimizing a servo track writing process in which servo tracks are separately written on each disc a single disc at a time.
In a hard disc drive magnetic data storage device, servo data and user data are magnetically written and read from magnetic tracks laid out on an upper and/or lower surface layer deposited on a flat rotating disc in the drive. Accurate location of the tracks and accurate positioning of the read/write heads carried by an actuator assembly is critical to the optimal operation of these drives. The concentricity of the tracks with respect to the rotational center of the drive motor spindle is paramount to facilitating closer and closer track spacing as the demands for higher densities of data storage increase with each generation of disc drives. However, there is always some error present in the concentricity of the tracks with respect to the disc center and the drive motor spindle center. The non-concentricity error is termed Once Per Revolution (OPR) positional error.
There are two basic methods of writing servo tracks on these drives. The first method is to write the servo tracks in a servo track writer (STW) on each disc individually prior to drive assembly, separate from the disc xe2x80x9cpackxe2x80x9d, and then stacking the discs on the drive motor spindle to assemble the pack and the overall drive. This method requires a separate, single disc servo track writer, into which each disc is placed and written. The second method is to assemble the drive motor onto the drive base plate, the discs onto the drive motor spindle in the drive into a disc xe2x80x9cpackxe2x80x9d, and then insert the partially assembled drive into a xe2x80x9cPack-writerxe2x80x9d to write the servo tracks on the discs in the pack. This pack-writer utilizes the drive""s actuator assembly to write the tracks and utilizes apertures in the disc drive housing in conjunction with a laser interferometer to index and position the heads or a push pin contact with an external positioner to position the heads during the STW process.
This latter, pack-writing process is an extra step in the drive manufacturing sequence that can be avoided if the servo tracks on the discs are previously written in the single disc STW process. One problem in single disc STW process, however, is the compounding of OPR positional errors which are introduced because of the manufacturing tolerances in the inner diameter dimensions of the disc and the outer diameter tolerances of the drive motor spindle. As each disk is stacked on the drive motor spindle there can be an error in concentricity of the disc with the drive center. Further, during the single disc STW process, there is a dimensional tolerance between the disc inner diameter (ID) and the STW spindle outer diameter (OD). These two different tolerances can introduce compounded error in the concentricity of the written servo tracks with respect to the disc center in the actual disc drive. Further, when several discs written by the single disc STW process are stacked on a drive spindle during drive manufacture, these non-concentricity errors or OPR errors can further compound such that the peak to peak OPR error in the assembled stack can be several mils or more.
The single disc STW uses the STW spindle center as the base reference for determining the track locations. A single disc STW can generate errors in track writing once per revolution (OPR) errors from head to head across 100""s of tracks. These errors compound when such written discs are assembled into an actual disc drive. Therefore there is a need for a method for accurately reducing the potential peak to peak OPR errors that can result when single disc STW process is used.
The method and apparatus in accordance with the present invention addresses the above identified need. The method involves clamping a disc to be written in a single disc servo track writer, sensing the location of an outer edge portion of the disc, measuring a once per revolution position error peak to peak amplitude value and phase value for that disc, determining an average OPR positional error therefrom, and adding this average OPR error to the baseline outer edge dimension to provide the baseline for locating the first written track and each subsequent track recording head position.
This method reduces the written in non-concentricity to the measurement and track writing servo""s accuracy. The OPR peak to peak amplitude and phase measurement establishes the disc average circular dimensions in the STW and makes all the STW written tracks concentric with the average dimensions. Then, when the disc is mounted on an actual disc drive spindle, only the tolerance errors between the stacked discs contribute to the OPR positional error of the tracks in the drive.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.